Yellow Squash Casserole With Easy Cheesy Cream Sauce

Name: Yellow Squash Casserole With Easy Cheesy Cream Sauce
Ingredients: 6 Tbsp. unsalted butter, divided, plus more for greasing dish; 3 lb. yellow summer squash (5–6 medium), sliced crosswise ¼ thick; 3½ tsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, divided; 1 medium onion, finely chopped; ¾ cup heavy cream; ½ cup sour cream; 4 oz. sharp white cheddar, grated (about 1 cup); 1 oz. Parmesan, finely grated (about ⅔ cup), divided; 1 Tbsp. thyme leaves, plus more for serving; ½ tsp. freshly ground black pepper; 1 sleeve Ritz crackers or other buttery crackers (about 32); Zest of 1 lemon